Electrochromism describes the induction of a color change in a medium as a result of charge transfer or electron transfer caused by an externally applied potential. The color changes are indications of induced chemical changes in the species of interest. For most chemical species exhibiting this effect, the change is from one color to another. As an example, viologen dye molecules change from yellow-orange to blue when reduced at a cathode. J. Bruinik, C. G. A. Kregting, and J. J. Ponjee, J. Electrochem. Soc. 124, 1854 (1977). Solid films of WO.sub.3 also show electrochromism with transparent films becoming blue upon reduction.
In order for electrochromic materials to be useful for display purposes, they must have optical absorption in the visible spectrum and exhibit mixed conduction capability (i.e. electronic and ionic). It is also highly desirable to exhibit high contrast from the background in order to modulate ambient light. Electrochromic materials generally have these properties. Electrochromic materials are usually operated with low voltages and can provide suitable contrasts with charge transfer of only several millicoulombs of electrical charge per square centimeter of display area. Erasure is easily made by polarity changes. These materials may also have the ability to hold images for the required response time of the human eye (about 0.1 second) and this further may allow for the use of memory effects. A major disadvantage of electrochromic displays is the lifetime of the device. Chemical degradation frequently occurs as usage time increases.
This disclosed invention is an improvement of the inventions described and claimed in U.S. Pat. No. 4,586,792 and co-pending application Ser. No. 858,384 filed 1 May 1986 both of which are incorporated by reference in their entireties in this disclosure. Further, a discussion of both the prior art generally and polyaniline is set forth in those documents. Also, the following publications by the inventor discuss the structure and electrochromism of polyaniline. McManus, P. M., Yang, S. C. and Cushman, R. J., Electrochemical Doping of Polyaniline: Effects on Conductivity and Optical Spectra, J. Chem. Soc, Chem. Commun., 1985, pp. 1556-1557; Cushman, R. J., McManus, P. M., Yang, S. C., Protonation and electrochemical doping of polyaniline: Correlation between the changes in electrical conductivity and optical spectrum, Makromol Chem., Rapid Commun. 8, pp. 69-75 (1987); Cushman, R. J., McManus, P. M. Yang, S. C., Spectroelectrochemical Study of Polyaniline: The Construction of a pH-Potential Phase Diagram, J. Electroanal. Chem. 291, pp. 335-346 (1986); and Cushman, R. J., McManus, P. M., Yang, S. C., Influence of Oxidation and Protonation of the Electrical Conductivity of Polyaniline, The Journal of Pysical Chemistry, Vol. 91, No. 3, pp. 744-747 (1987). Morphological Modification of Polyaniline Using Polyelectrolyte Template Molecules, Hwang, J. H., Yang, S. C., Synthetic Metals, Vol. 29, P. P. E271-E276, (1989).
The display elements of the prior invention overcame to some extent the prior art problems of longevity and the inability of the films to repeatedly produce color changes, including transparent, which are necessary for the successful application of electrochromism in electronic color display devices. Further advantages of those inventions were a display screen in a thin plate or rollable sheet which consumed a minimal amount of electrical power. Most importantly, a multicolor display was achieved which capability was not available in then available liquid crystal display devices.
One of the prior inventions was directed to the use of the polymeric aniline between two transparent surfaces for use such as in windows, windshields, glasses, bowls, decorative panels and the like. The invention embodied multi-color switchable panels which responded rapidly and changed colors and tints with rapidity and also became completely transparent. The prior inventions disclosed the use of both solid and liquid electolytes although the liquid electrolytes were disclosed for the preferred embodiment.
The present invention embodies (1) the use of solid electrolytes such as described in the original patent and as prepared in the prior referenced application, (2) the use of polymer composites of polyaniline/polyelectrolyte as electrochromic material, (3) the construction of a layered structure of the polyaniline/polyelectrolyte composite so that the type and the concentration of the polyelectrolyte in the composite are varied.
The function of the electrolyte in an electrochromic device is to serve as an ion-conductor to complete an electrical circuit. By allowing ions to migrate between the two outer electrodes, the electrolyte carries the electrical current that is necessary for the color switching. The electrolyte allows the ions to transport between the two plates but blocks the flow of the "free" electrons.
Solid electrolyte is more practical than liquid electrolyte for manufacturing electrochromic devices. In an electrochromic light filter or an electronic display, the electrolyte is sandwiched between two flat electrodes. When a liquid electrolyte is used, the two flat electrodes need to be sealed before the liquid electrolyte is introduced. The use of a thin film solid electrolyte renders the manufacturing process much easier because no such sealing is involved in making the sandwich structure.
The solid electrolyte films generally function as having two layers, an ion storage layer and an ion conductor layer. The ion conductor layer interfaces with the electrochromic layer. The color changes are effected by ions moving between the conductive layer and the electrochromic layer. The ions entering and leaving the electrochromic layer result in the color changes. Because the electrochromic and ion conductor layers have rigid lattice structures and must repeatedly expand and contract to accept and release the ions, ultimately mechanical strains develop and cracks and gaps are formed at the interface between the conductive layer and the electrochromic layer resulting in degradation of the color.
The mechanical contacts between the electrochromic material and the solid electrolyte are not achieved down to the molecular level. Two solids usually do not adhere well. For example, if a film of PbF.sub.2 solid electrolyte is pressed upon a Lu-diphthalocyanine film coated on tin oxide glass by mechanically contacting the two films, the electrical resistance is prohibitively large for it to complete a circuit. At the microscopic level, only a small fraction of atoms are making molecular level contacts.
There have been attempts to make closer contact between the two films by molecular deposition. The electrolyte molecules are vapor deposited onto the electrochromic film in a slow process where the thickness of the electrolyte is built up from monomolecular layers. For example the solid electrolyte PbF.sub.2 is evaporated onto a Lu-diphthalocyanine electrochromic electrode. In this manner the solid electrolyte grows by molecular layers instead of by chunks. The molecules of the solid electrolyte PbF.sub.2 fill into the microscopic cracks and the holes of the electrochromic phase (Egashira et al. Proceedings of the Society for Information Display, 28 227 (1987)). The films built up in this way have fewer gaps in the interface than when two layers are pressed together.
The problems with the vapor deposition process are two fold. It is a more costly process than laying down the prefabricated polymer electrolyte films; and the good static contact is soon lost after a number of switching cycles. This is because during the color switching cycles, the dimension of the lattice changes in each cycle. The flouride ion is inserted and removed from the interface during each cycle. The dimensional changes are not reversible enough to maintain the original mechanical contacts at the interface.
In the prior art, polymer electrolytes are believed to have been used for only one of the two phases at the interface. For example, the solid electrolyte was a polymer electrolyte but the electrochromic material was an inorganic solid which has a rigid lattice (T. Katsube, M. Hara, T. Yaji, S. Kobayashi, K. Suzuki, and Y. Nagawa, Proceedings of the Society of Information and Display, Vol. 28/3 (1987), pp. 233-237). The problem of lattice expansion/contraction cycles still remains in the electrochromic layer. The severity of the problem is partially lessened by using smaller ions (H.sup.+ and Li.sup.+) as the dopant which shuttles across the interface. But the problem is not totally solved. In addition the inorganic solid electrochromic materials do not share some of the advantages of the organic polymer electrochromic materials.
The disclosed invention overcomes the prior art problems of both rigidity and a planar interface between the electrochromic film and the electrolyte film. The electrochromic material of the invention is polymerized `in situ` in a polymeric electrolyte to form an electrochromic/polyelectrolyte mixture. This mixture is coated as a film on electrochromic material. A film of electrolyte is placed in ion transfer relationship with the electrochromic/polyelectrolyte film. This electrochromic/polyanaline/polyelectrolyte film greatly increases ion transfer between the electrochromic film and the electrolytic film by interposing a region of molecularly mixed electrochromic material and electrolyte.
In the preferred embodiment of this invention, the film of electrolyte is itself a polymer so it can be formed as a thin plastic film. State of the art processes for making laminated glass or plastics can be adapted easily. The polymer electrolyte can also be dissolved in solvents and painted or sprayed onto the electrodes and be fixed by evaporating the solvent as disclosed in the previously cited application.
This invention involves an improvement over the prior art for electrochromic light filters. The improvement is realized by using compatible or common polymers for both the electrochromic and the electrolyte films to form polymer-based junctions at the molecular level between the electrochromic and electrolyte films. The improvements result in lower manufacturing costs and the devices have better switching characteristics and a longer life span.
The term "solid electrolyte" for purposes of this disclosure includes, in addition to normally solid materials, viscous materials, as long as the fluidity of the electrolyte alone or in combination with the electrochromic material is low enough to remain dimensionally stable during manufacture and use of the electrochromic device. It is conceivable that a viscocity comparable to glycerol (greater than 10 poise) is sufficient for gaining the desired advantages in manufacturing.
Solid polymer electrolytes are used that do not have rigid lattices. The conformation or the structure of the host molecules (electrolyte) is flexible enough so that it reversibly expands and contracts, in synchronization with the ion movements. The flexibility of the polymer molecules allows the dynamic strain be relieved.
Preferred solid electrolyte includes but are not limited to;
Poly-(vinylsulfonic acid) or its salts, [--CH.sub.2 CH(SO.sub.3 H)--].sub.n ; PA0 Poly-(acrylic acid), [--CH.sub.2 CH(CO.sub.2 H)--].sub.n ; PA0 Poly-(styrene sulfonic acid) [--CH.sub.2 CH(C.sub.6 H.sub.4 SO.sub.3 H)--].sub.n PA0 Poly-(2-acrylamido-2-methyl-1-propane-sulfonic acid), (--CH.sub.2 CH[CONHC(CH.sub.3).sub.2 --CH.sub.2 SO.sub.3 H--].sub.n ; PA0 Poly-(ethylene oxide), (--CH.sub.2 CH.sub.2 O--).sub.n ;
Polyphosphazenes, A family of polymers with polyphosphzene backbones and polyether side groups. For example: ##STR1##
Polyelectrolytes and gel electrolytes are polymers that have either cationic or anionic groups chemically bonded to a polymer chain. Among the group of chemicals cited in the above polyacrylic acid, polystyrene sulfonic acid and some others are examples of polyelectrolytes. When these polymers are cross linked they can form gel electrolytes. In the electrochromic devices, the polyelectrolytes are mixed with a small amount of high dielectric constant solvent (such as water or propylene carbonate) so that the counter ions of the polyelectrolyte can migrate within the matrix formed by the polymer chains. If gel electrolyte is used, small amounts of polar solvent and salt are added to increase the ion conductivity. Usually about 1% by weight of the solvent or salt is sufficient to maintain ionic conductivity for thin films of solid electrolyte layer.
The invention, in one aspect, is a composition of the polymeric aniline molecularly mixed with solid polymer electrolytes. In another aspect, the invention embodies the mixture in combination with an electrochromic device.
The inventive improvements involve better mechanical stability of the electrochromic device and better cycling lifetimes.
To achieve these improvements, the electrochromic material, polyaniline, is synthesized in a solution of polyelectrolyte so that the morphology of polyaniline is controlled to provide good contact with the solid electrolyte layer of the electrochromic device. Furthermore, the condition for synthesis is adjusted so that suitable composite is formed from polyaniline and some electrolytes. The composite polymer provides further advantages for good mechanical strength and better ion exchange properties .